White dwarf: The death-bed of a star telling the story of its glorious past

 

A white dwarf is a compact stellar core remnant resulting from gravitational collapse. Dense electron degenerate matter fills up such astronomical objects. Since nuclear fusion ceases to take place in a white dwarf, it almost has no light of its own. But still, a white dwarf may appear faintly lit in the sky, due to the residual thermal energy that it carries forward from the fusion reaction during the star's lifetime. Sirius B at a distance of 8.6 lightyears is the nearest known white dwarf. Around 8 white dwarfs are known to us in the vicinity of our solar system.

Formation

When a star is on the verge of completing its life cycle, its thermonuclear fuel is completely depleted from the core. This results in the halt of the nuclear fusion occurring in its core and there is an imbalance of outward and inward forces in the star with the inward gravitational force dominating the outward thermonuclear force. So the star collapses due to its own mass, with its volume becoming smaller and smaller. This makes the inside material denser and denser. Now if the original star does not have enough mass (> 25 solar masses) to form a neutron star, the remnant generally goes on to form a white dwarf, which is the final evolutionary stage of such stars.

In the main sequence, a star fuses hydrogen to form helium, releasing an enormous amount of energy that counterbalances the inward gravitational attraction. But when all the hydrogen gets burned up, the star expands into a red giant phase, when it fuses helium into carbon and oxygen (true for low and medium mass stars). Now if the red giant cannot produce the necessary temperature (around 1 billion K) to fuse carbon, then the core will be filled with inert carbon and oxygen. After shedding off the outer layers, forming a planetary nebula, the final remnant core rich in inert carbon and oxygen will be the white dwarf. This is the most general scenario and most abundantly found in the universe. But in the case of some small stars, they are not able to fuse helium into carbon and oxygen, and so they form helium-rich white dwarfs.

Stellar Equilibrium

At the white dwarf stage, the further collapse of the star is hindered by the electron degeneracy pressure. But there is a maximum mass limit that can be supported by this degeneracy pressure. This limit is called the Chandrasekhar limit which is approximately equal to 1.44 solar masses. So if a white dwarf accumulates enough matter from its companion star to go past the Chandrasekhar limit, then the electron degeneracy pressure can no longer hinder further gravitational collapse. The resulting white dwarf explodes into a very violent and bright event known as a supernova and the core further collapses towards the formation of a neutron star.

Transformation into a black dwarf and the final fate

Since there is no nuclear fusion going on in a white dwarf, it slowly radiates out whatever remnant heat it has over time and cools down. So initially white dwarfs are bluish-white (hot) due to the radiations emanating from them, but with time they become redder due to cooling. Finally, these dwarf stars are supposed to totally stop radiating and turn into black dwarfs. But the time taken for a white dwarf to turn into a black dwarf is calculated to be greater than the age of the universe, and hence it is thought that there are no such black dwarfs existing in the universe today. The oldest white dwarfs exiting today still radiate out the heat of a few thousand kelvins, which is such a mind-blowing fact!! Even if a black dwarf forms, it will be a really challenging task to locate it due to its non-radiating nature. 

No matter how dark, cool or insignificant they are, such entities will continue to roam around in the eternal space and tell the stories of their glorious past!! Such death beds of once fierce and powerful stars will keep whispering through the ages!!

By

Prabir Rudra